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Parametric Determination of Minimum Streamflow for Trout


The approach taken in the determination of a suitable minimum flow to be maintained in a stream channel for the preservation of trout populations has centered on three types of physical stream characteristics and the changes observed in them between various discharge levels, based on the average daily flow (ADF) over the period of record: 1) hydrologic parameters; 2) surface area and its composition, based on water depth and velocity; and, 3) available trout cover.

Portions of Douglas Creek and Hog Park Creek, relatively small streams (average dally flows approximately 30 cubic feet per second) located in the North Platte River drainage of southeastern Wyoming, were intensively investigated in the summer and fall of 1972 at 200%, 100%, 50%, 25%, and 12.5% ADF. Water depth, velocity, crosssectional area, wetted perimeter, hydraulic radius, top width, total surface area, surface area having a velocity of at least 1.0 feet per second, surface area of depth 0.5 feet or greater, and available brown trout cover were found to decrease at the greatest rate for the discharge reduction interval from 25% to 12.5% ADF. As a minimum flow, a discharge in the 25% ADF range will avoid the flow range for which the rate of habitat decrease is greatest.

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